In recent years, there continues to be dramatic density increases in integrated circuit technology for semiconductor chips. For example, the minimum feature size of lithography, such as the size of MOSFETs, has presently been reduced to one micrometer and below. Many applications implemented on modern semiconductor integrated circuit (IC) chips require accurate voltages, which becomes increasingly difficult to provide as chip density continues to increase. To provide these accurate, regulated voltages, precise and constant reference voltage signals must be generated and maintained during circuit operation.
Making the task of generating constant reference voltages more difficult are several on-chip and environmental effects that consistently counteract the regulation of on-chip voltages. Examples include temperature effects and manufacturing process variations with the structures of the components creating the reference voltage generator circuit. Relatively extreme variations in temperature, for example, the operating temperature of active devices within the generator circuit, often affect the resistance, capacitance, and voltage, and thus the current flow, of on-chip components, which affects the operation of the IC chip itself. More specifically, such process variations typically affect line spacings and the thickness of oxides, metals, and other layers of the semiconductor wafer, which consequently can affect on-chip voltages.
Initial approaches to provide circuit capable of generating substantially constant reference voltages in spite of these environmental effects have included the use of bipolar junction transistors (BJTs). While such BJT circuits typically provide adequate compensation for temperature-based circuit variations, they do so at the expense of large current draws (due to operation in the active region), as well as occupying large areas of valuable chip real estate. Other conventional approaches have been made using MOS components operated in the weak inversion state to obtain a stable PTAT voltage. One example is found in the paper entitled, “Optimal Curvature—Compensated BiCMOS Bandgap reference” by Popa and Mitrea. However, the current level of the MOS transistor in the weak inversion state is too low to get a stable reference voltage in the environment like a high density DRAM where a large internal noise is induced during operation. In addition, the MOS model in the weak inversion mode is typically not advantageously used safely in such a circuit design.
Other conventional approaches have operated the MOS components in active mode operation, in order to overcome the drawbacks of the weak inversion component operation. An example may be found in the paper entitled, “A Precision CMOS Voltage Reference with Enhanced Stability for the Application to Advanced VLSI's” by Yoo, et al. Unfortunately, while such an active mode operation approach does often offer a stable reference voltage in spite of temperature fluctuations, this approach does not seem to solve stability problems associated with process variations of the MOS components themselves. Accordingly, a more advantageous reference voltage generating circuit is desired.